1. Field of the Invention
The present invention relates to a freezing detection method for a fuel cell. More particularly, the present invention relates to a freezing detection method for detecting a change in one of an output voltage of a fuel cell, an output current thereof, cell resistance, cell temperature, and cell fastening pressure to enable more prompt and reliable detection of the start of the freezing of water within a fuel cell unit compared with a conventional detection method.
2. Description of the Related Art
When a fuel cell is used under a low-temperature environment, water generated within the fuel cell may freeze. Depending on a place where freezing occurs, reaction gas intake becomes difficult, which leads to problems such that a sudden output drop gives damage to a connection device, and adversely affects a member constituting the fuel cell. Accordingly, in the case of mounting the fuel cell in an automobile and a portable device which may be used under a low-temperature environment, there is required a method which enables, when water within the fuel cell starts to freeze, prompt and precise detection of freezing within the fuel cell before the whole fuel cell freezes.
As a method for preventing the freezing of a fuel cell, there is a conventional method which constantly monitors an outside air temperature, and activates a heating unit when the outside air temperature reaches a level under a freezing point. However, because a fuel cell generates heat through power generation, the fuel cell does not freeze momentarily after reaching a level under the freezing point. In addition, because an amount of generated heat depends on the situation of the time in which power is generated, a temperature at which the freezing starts sequentially varies. That is, in the method, the heating unit is activated even in a non-freezing situation, an energy loss increases. Accordingly, an energy efficiency can be increased if the method is improved so as to take measures after detecting the start of freezing. For example, as described in Japanese Patent Application Laid-Open No. 2002-313391, there is proposed a method in which a heating unit is activated when an output voltage is low, and the heating unit is halted when the output voltage is high. On the other hand, Japanese Patent Application Laid-Open No. 2005-142022 proposes a method in which a fuel cell is determined to freeze when, at an outside air temperature under the freezing point, a cell resistance gradually decreases before abruptly increases.
However, as the cause of changes in cell resistance and output power, various factors can be considered, such as load fluctuations, a dry-out phenomenon in which an electrolyte membrane dries, and a flooding phenomenon in which generated water clogs a supply path for a reaction gas.
That is, with the conventional technology described above, it is difficult to distinguish phenomena other than freezing as described above from real freezing so that it is difficult to precisely detect freezing. Consequently, the conventional technology had the problem that waste of energy may occur due to a malfunction such as the activation of a heating unit in the event of a phenomenon other than freezing.